Resonant and semi-resonant DC-DC converters, including isolated and non-isolated topologies, are used in a variety of applications including telecommunications, consumer electronics, computer power supplies, etc. The usage of such converters is gaining popularity because of their zero-voltage switching (ZVS) and/or zero-current switching (ZCS) characteristics, and their ability to utilize parasitic electrical properties inherent in an electronic circuit. Among numerous topologies, the semi-resonant converter with transformer/tapped inductor is an attractive topology for providing high voltage-conversion ratios without requiring a high number of components. Such converters provide advantages including lower cost and higher efficiency as compared to other solutions.
One class of semi-resonant converters includes a power stage with high-side and low-side switches that transfer power from an input source to a tapped inductor that supplies output power to a load. The tapped inductor is also connected to a second low-side switch, which is termed a synchronous rectification (SR) switch herein. In order to meet the power requirements for a load of a semi-resonant converter (e.g., provide a near constant output voltage for the load), many semi-resonant DC-DC converters employ a variable switching frequency wherein the switching period can vary from cycle to cycle. During a portion of each switching period, the SR switch will be enabled such that current flows through it. For the semi-resonant converter described above, the current during this portion of a switching period will be shaped like one half cycle of a sinusoidal period.
Unlike other types of switching power converters, resonant and semi-resonant DC-DC converters such as the resonant tapped inductor converter can react to load changes much faster than the voltage/error changes. However, with high Q double poles at the switching frequency, the voltage loop of such a converter cannot be designed to have high enough bandwidth. Accordingly, the reaction of the converter to a dynamic transition in the load voltage is very slow with remarkable latency.
Dynamic transitions in the load voltage are typically handled by reshaping the target voltage and forcing the output voltage to move in a desired direction. However, such approaches may not result in an optimal transition response in some resonant and semi-resonant DC-DC converters. In addition to the loop bandwidth limitation issue, the shape of surge current needed to charge the output capacitor of the converter is different compared to other types of converters, so using an offset may not result in optimal surge current cancellation in AVP loop.
Accordingly, there is a need for improved dynamic voltage transition techniques for resonant or semi-resonant DC-DC converter that use synchronous rectification (SR) switches.